Our structural efforts are driven by the following questions to explore the biology of peptide receptors of the class A (rhodopsin) GPCR family, using NTSR1 as a model system. (i) How do peptide ligands interact with their respective peptide receptors? (ii) How does the binding of a peptide ligand translate into the structural changes within the receptor to allow the activation of the G protein (iii) How is specificity of G protein binding achieved? We have determined at 2.8A resolution the x-ray crystal structure of NTSR1 bound to its peptide agonist in an active-like state. A number of strategies were implemented to achieve this: (i) Conformational thermostabilization generated a NTSR1 mutant with greatly improved stability, locked into an agonist binding, active-like conformation. (ii) The use of the T4 lysozyme technology replacing the third inner loop promoted crystal contacts. (iii) Lipidic cubic phase crystallization resulted in highly diffracting crystals. The NTSR1 structure has many hallmark features of an active-like receptor conformation such as an outward-tilted transmembrane helix 6 at the cytoplasmic surface and key conserved residues in positions characteristic for active but not for inactive GPCRs. The neurotensin binding pocket is wide open on the extracellular surface of NTSR1. The peptide agonist NTS8-13, the C terminal portion of neurotensin responsible for agonist-induced activation of the receptor, binds to NTSR1 in an extended conformation nearly perpendicular to the membrane plane with the C-terminus oriented towards the receptor core. There is a striking difference between the binding mode of NTS8-13 in NTSR1 and the binding of small agonists in rhodopsin or beta-adrenergic receptors. NTS8-13 does not penetrate the receptor as deeply compared to those small agonists, indicating that the mode of activation of NTS1 is subtly different from these receptors. More recently, additional structures of NTSR1 have revealed how binding of the agonist peptide relates to conformational changes important for activation, and we have defined the structural prerequisites for NTSR1 signaling. Most recently, we have determined the structure of a constitutively active NTSR1 mutant, which signals in the absence of an agonist. However, the agonist-induced activation of the G protein is reduced, as found with other constitutively active GPCRs. This provides the unique opportunity to assess by structural and pharmacological aspects, and by molecular dynamics simulations (collaboration with N. Vaidehi) the features promoting constitutive activity, and why the agonist peptide is not effective to stimulate a signaling response to the level of the wild-type receptor. As our goal is to understand the molecular events that occur upon signaling of NTSR1, we pursue crystal structures of NTSR1 in several signaling states. Specifically, we will obtain structures of NTSR1 in the inactive state and the G protein-bound states, which will allow a detailed comparison of the similarities but importantly also differences between the signaling properties of GPCRs, which bind small molecule ligands deep within their transmembrane cores, and peptide receptors, which bind their ligands at the receptor surface.